The present invention relates to the hot forming of non-ferrous metals, and more particularly relates to the continuous casting and hot forming of the as-cast bars of certain impure copper or aluminum alloys which are otherwise prone to crack during hot-rolling, i.e. exhibit "hot-shortness".
It is well known that many common non-ferrous metals, such as copper, may be continuously cast, either in stationary vertical molds or in a rotating casting wheel, to obtain a cast bar which is then immediately hot formed, while in a substantially as-cast condition, by passing the cast bar exiting the mold to and through the multiple roll stands of a rolling mill while the cast bar is still at a hot-forming temperature. It is also well known that the as-cast structure of the metal bar is such that cracking of the cast bar during hot forming may be a problem if the cast bar is required to be directly hot formed into a semi-finished product, such as redraw rod, during which the initially large cross-sectional area of the cast bar must be substantially reduced, by a plurality of deformations along different axes, to provide a much smaller cross-sectional area in the product. See for example, the discussion of cracking U.S. Pat. No. 3,349,471.
While this problem could be avoided by casting a cast bar having an initially small cross-sectional area which need not be substantially reduced to provide the desired cross-sectional area of the final product, this approach is not commercially practical since high casting outputs, and therefore low costs, can be readily achieved only with cast bars having large cross-sectional areas which are rapidly reduced to the smaller cross-sectional areas of the products, such as 3/8" diameter rod for drawing into wire, by a minimum number of severe deformations. Thus, the problem of a cast bar cracking during hot forming must be solved within the commercial context of cast bars having initially large cross-sectional areas which are then hot formed into products having small cross-sectional areas by a minimum number of reductions which often are substantial enough to cause cracking of the cast bar under certain rather common conditions as discussed below.
This problem has been overcome in the prior art for relatively pure electrolytically-refined copper having low impurity levels such as 3-10 ppm lead, 1 ppm bismuth, and 1 ppm antimony. For example, U.S. Pat. No. 3,317,994, and U.S. Pat. No. 3,672,430 disclose that this cracking problem can be overcome by conditioning such relatively pure copper cast bar by initial large reductions of the cross-sectional area in the initial roll stands sufficient to substantially destroy the as-cast structure of the cast bar. The additional reductions along different axes of deformation, which would cause cracking of the cast bar but for the initial destruction of the as-cast structure of the cast bar, may then safely be performed. This conditioning of the cast bar not only prevents cracking of the cast bar during hot forming but also has the advantage of accomplishing a large reduction in the cross-sectional area of the cast bar while its hot-forming temperature is such as to minimize the power required for the reduction.
The prior art has not, however, provided a solution to the cracking problem described above for many metals, such as fire-refined scrap copper, containing a relatively high degree of impurities, or for initially pure cathodic copper which has become contaminated, during the melting and casting process, with a high volume fraction of intermetalic oxides and/or gas bubbles. This is because the large amount of impurities collect in the grain boundaries of the coarse as-cast structure and cause the cast bar to crack when an attempt is made to substantially destroy this coarse as-cast structure with the same large initial reduction of the cross-sectional area of the cast bar that is known to be effective with low impurity metals. Moreover, the greater the percentage of impurities in the cast bar, the more likely it is that cracks will occur during hot rolling. This problem is sometimes called "hot-shortness".
Thus, although there is no requirement for high-purity electrolytically-refined copper (except for specialized uses such as magnet wire) it has heretofore been necessary to use such highly refined copper in order to be able to use and obtain the many advantages of tandem continuous casting and hot-forming apparatus. As a result, a substantial refining cost is added to the price of many final copper products even though high purity is not required to meet conductivity or other physical specifications. For example, fire-refined copper wire having a moderately high degree of impurities (such 25 ppm Pb & Sn & 20 ppm Fe, etc.) can meet the IACS conductivity standard for household electrical wiring and can be produced more economically (since the copper cost is about 10 to 20% less) if the rod to be drawn into such wire can be produced using known continuous casting and hot-forming apparatus.